An air-slot photonic crystal cavity is studied based on the design of an air-slot in a width-modulated line-defect in a photonic crystal slab. The electric field of the cavity mode is strongly localized in free space. Owing to the discontinuity of the dielectric constant, the electric field of the cavity mode is strongly enhanced inside the slot and the mode volume is strongly compressed. The cavity band structure and cavity modes of this air-slot photonic crystal cavity are numerical simulated and analyzed. According to the resonant frequency and the symmetry of the cavity modes, the first order even mode has both high quality factor and small cavity volumn. Using finite-difference time-domain method, the calculated quality factor is as high as 106 and the mode volume is as small as 0.02 of a cubic wavelength in a vacuum. Additionally, the properties of the first order even cavity mode as a function of the slot width and the radius of the holes are calculated. The resonant frequency of the first order even mode is decreased with the solt width and radius of the holes. But when the radius of the holes are 170 nm, the highest quality factor of the air-slot photonic crystal cavity is obtained.